Trimethylglycine, also commonly known as TMG, is an organic compound described by the formula.

Trimethylglycine is used to treat high homocysteine levels.[1] Kilmer S. McCully, MD, theorised that cholesterol and clogged arteries were symptoms rather than causes of heart disease and proposed homocysteine as a more likely culprit. If it were not for his work, homocysteine would not have been thought harmful and so supplements to lower homocysteine would not have been thought necessary. Betaine hydrochloride ("betaine HCl") is the chloride salt of TMG. Used as a digestive aid, the hydrochloride is particularly helpful for persons with insufficient acid production in the stomach. Betaine HCl has an acidic taste whereas anhydrous TMG ("anhydrous betaine") tastes sweet with a metallic aftertaste and is usually produced from sugar beets (as is betaine hydrochloride). Both are active as methyl donors, as 'betaine' is retained in both forms. Betaine HCl (hydrochloride) has a chloride counterion, and is otherwise identical, chemically, to TMG.

TMG functions very closely with choline, folic acid, vitamin B12 and S-adenosyl methionine SAMe. All of these compounds function as methyl donors. They carry and donate methyl functional groups to facilitate necessary chemical processes. The donation of methyl groups is important to proper liver function, cellular replication, and detoxification reactions. TMG also plays a role in the manufacture of carnitine and serves to protect the kidneys from damage. Trimethylglycine / betaine donates a methyl group to convert homocysteine to methionine in a reaction catalysed by BHMT (Betaine Homocysteine Methyltransferase, E.C. 2.1.1.5, a zinc metalloenzyme). Methionine is then converted to SAMe by Methionine Adenosyl Transferase (MAT) using magnesium and adenosine triphosphate as co-factors.

TMG is related to choline (trimethylaminoethanol), with the difference that the terminal carboxylic acid group of trimethylglycine has been reduced to a hydroxyl group in choline. The product of demethylation of TMG is dimethylglycine. Alkylated derivates of trimethylglycine have uses as quaternary ammonium zwitterionic surfactants. TMG (trimethylglycine), also known as anhydrous betaine, is found in a variety of plant and animal sources and is used in the conversion of homocysteine to methionine. Maintaining normal homocysteine levels is important for the health of the cardiovascular system. TMG has been shown to help protect the liver and raise S-adenosylmethionine (SAM-e) levels, in animal studies. Trimethylglycine TMG (also known as betaine), and Dimethylglycine DMG, are methyl donors that help in the production of several brain chemicals and hence improve mood, energy, wellbeing, alertness, concentration, and visual clarity.

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